The present invention relates to an X-ray diffraction contrast tomography (DCT) system comprising an X-ray source for providing an X-ray beam in a direct path; a staging device for positioning and rotating a polycrystalline material sample in the direct path of the X-ray beam; a first X-ray detector located in the direct path with the staging device positioned between the first X-ray detector and the X-ray source, allowing said first X-ray detector to detect a direct X-ray beam being transmitted through the crystalline material sample; and a processing device for analysing detected values and determining crystallographic grain centre-of-mass positions and grain orientations in the polycrystalline material sample.
Such a DCT system is described in the article “X-ray diffraction contrast tomography: a novel technique for three-dimensional grain mapping of polycrystals. Part 1: direct beam case” published in Journal of Applied Crystallography (2008), 41, 319-326. A synchrotron X-ray beam is used to illuminate the sample, and the X-ray detector detects a combined absorption contrast and diffraction contrast image of the transmitted beam. By subtracting the absorption contrast part, it is possible from the remaining diffraction contrast to perform tomography of grains embedded in a polycrystalline mono-phase material. With traditional absorption or phase contrast tomography only the outer contour of a mono-phase specimen could be detected. With X-ray diffraction contrast tomography, the grains of the polycrystalline material sample under examination are imaged using the occasionally occurring diffraction contribution to the X-ray attenuation coefficient in the non-diffracted X-ray beam leaving the crystalline material sample. Each time a grain fulfils the Bragg diffraction condition a diffraction contrast occurs. The diffraction contrast appears on the detector behind the sample as an extinction spot caused by a local reduction of the transmitted beam intensity recorded on the detector. In the article, the three-dimensional grain shapes are reconstructed from a limited number of projections using algebraic reconstruction techniques (ART). The procedure for the three-dimensional grain shape reconstruction is based on spatial filtering criteria only, and the procedure can therefore be performed without analysing the grain orientations. With respect to grain orientations the article specifies that the intensity of the diffractions spots must be included in order to determine orientations, and even with integrated intensities several solutions may exist and choices have to be made. It is explained in the article that overlapping diffraction contrasts present a problem and that the sample consequently had to have only little grain orientation spread, grains of approximately the same size and tailored transverse sample dimensions in order to limit the probability of spot overlap.
Considerable efforts have been put into the development of techniques for three-dimensional grain mapping of polycrystalline materials. These techniques are utilizing X-ray beams from a synchrotron facility and employ reconstruction algorithms of the kind known in tomography in order to provide a non-destructive characterization of a sample of polycrystalline material.
Conventional X-ray absorption tomography systems as disclosed in U.S. Pat. No. 5,245,648 are able to provide a spatial representation of a sample of crystalline material. However, the contrast of such spatial representation is based on the densities of the sample and if the sample has a homogeneous density, it therefore only represents the periphery of the sample. Furthermore, if the sample comprises several phases of different materials, the internal structure within each of these phases is not visible in the representation. Moreover, the densities do not provide information of the grain structure of a crystalline material sample and hence no information about the orientation (or stress) of the grain lattice, which is equally important to determine as the spatial representation of the sample.